Optical tomographic imaging apparatus

ABSTRACT

Provided is an optical tomographic imaging apparatus which enables simplification and cost reduction without reducing accuracy when moving part of an object is moved in an optical axis direction of measuring beam. The apparatus using return beam of measuring beam reflected or scattered by an object and reference beam reflected by a reference mirror to image the tomographic image, includes: a reflecting position controlling device for controlling the reflecting position of the reference mirror; a detecting device for a position in a moving part having an optical system for observing the moving part illuminated by an optical system imaging the same on an area sensor based on the Scheimpflug principle and detects position information that the moving part is moved in the direction; and a device for driving the reflecting position controlling device to control the optical path length of the reference beam based on the position information.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical tomographic imagingapparatus. More specifically, the present invention relates to anoptical tomographic imaging apparatus used for ophthalmic diagnosis.

2. Description of the Related Art

At present, various ophthalmological apparatuses using opticalapparatuses are used. For instance, various optical apparatuses forobserving eyes, such as an anterior ocular segment photographing device,a fundus camera, and a scanning laser ophthalmoscope (SLO), are used. Inparticular, an optical tomographic imaging apparatus by an opticalcoherence tomography (OCT) using interference phenomenon ofmulti-wavelength light is an apparatus which can obtain a sampletomographic image at high resolution. The ophthalmological apparatus hasbecome indispensable for out patient department specializing in aretina. Hereinafter, this apparatus will be referred to as an OCTapparatus.

The OCT apparatus can irradiate measuring beam as low coherent light toa sample and, using an interference system, measure backscattering lightfrom the sample at high sensitivity. In addition, the OCT apparatus canscan the measuring beam on the sample and thereby can obtain atomographic image at high resolution. For this reason that the OCTapparatus can obtain a tomographic image of a retina in the fundus of aneye to be inspected at high resolution, the OCT apparatus is widely usedfor ophthalmological diagnosis of retina. Meantime, an eye of a humanhas an involuntary motion of an eye ball called an involuntary eyemovement. Therefore, if the time for the ophthalmological diagnosis of aretina using the OCT apparatus is longer, motion artifacts, which is anirregularity of an image, is caused in the obtained tomographic image ofthe retina due to the influence of the eye movement.

To prevent motion artifacts, various attempts have hitherto been made.For instance, the document of Christoph K. Hitzenberger, “SimultaneousSLO/OCT imaging of the human retina with axial eye motion correction”,Optics Express Vol. 15, No. 25 (2007) discloses a method of usinganother OCT apparatus in addition to an OCT apparatus for observingfundus. The another OCT apparatus is an apparatus which uses a lightsource of 1300 nm to obtain a tomographic image of a cornea and monitorsthe position of the cornea in an optical axis direction (hereinafter,called a vertical direction) of the OCT apparatus for observing fundus.A method has been proposed to control the reference mirror of the OCTfor observing fundus for measurement according to the position of thecornea in a vertical direction. The position of the cornea is calculatedusing the OCT in this manner to reduce motion artifacts to thetomographic image of the retina due to the influence of the eyemovement.

As described above, if the measuring time in fundus observation usingthe OCT apparatus is longer, image irregularity called motion artifactsdue to eye movement is caused in the obtained tomographic image of aretina. In the above document, the consideration for reducing motionartifacts is made. However, since the OCT for observing cornea is usedtogether with the OCT for obtaining the tomographic image of the retina,the apparatus becomes larger and the cost of the apparatus is greatlyincreased.

In view of the above problems, an object of the present invention is toprovide an optical tomographic imaging apparatus which enables anapparatus to be simplified and cost thereof to be reduced withoutreducing accuracy when position information that the moving part of anobject is moved in an optical axis direction of measuring beam isdetected to reduce the deformation of a tomographic image due toposition displacement in the moving part of the object.

SUMMARY OF THE INVENTION

The present invention provides an optical tomographic imaging apparatusconfigured as follows.

According to the present invention, there is provided an opticaltomographic imaging apparatus which splits light from a light sourceinto measuring beam and reference beam, guides the measuring beam to anobject, guides the reference beam to a reference mirror, and uses returnbeam of the measuring beam reflected or scattered by the object and thereference beam reflected by the reference mirror to image thetomographic image of the object, including: a reflecting positioncontrolling device for controlling the reflecting position of thereference mirror; a detecting device for detecting a position in amoving part which has an optical system for observing moving part of theobject illuminated by an optical system for illumination with the lightfrom the light source by imaging the same on an area sensor based on theScheimpflug principle and detects position information that the movingpart of the object is moved in an optical axis direction of themeasuring beam; and a device for driving the reflecting positioncontrolling device to control the optical path length of the referencebeam according to the position information detected by the detectingdevice for detecting a position in a moving part and reducing thedeformation of the tomographic image of the object due to positiondisplacement of the moving part of the object.

Further, according to the present invention, there is provided theoptical tomographic imaging apparatus, wherein the object is an eye, themoving part of the object is an anterior ocular segment, and thedetecting device for detecting a position in a moving part can detectthe positions back and forward the movement of the cornea of the eye inan optical axis direction of the measuring beam.

Further, according to the present invention, there is provided theoptical tomographic imaging apparatus, wherein when the depth resolutionof the optical tomographic imaging apparatus is Δ, the opticalmagnification of the optical system for observing moving part is β, andthe pitch of the area sensor is p, the following equation is satisfied:

Δ×β>p

Further, according to the present invention, there is provided theoptical tomographic imaging apparatus, wherein the optical systemconfigured in the optical path of the measuring beam and the opticalsystem for illuminating the moving part of the object in the opticaltomographic imaging apparatus are partially shared.

Further, according to the present invention, there is provided theoptical tomographic imaging apparatus, wherein the detecting device fordetecting a position in a moving part can derive the radius of curvatureR of the cornea by illuminating the center position of the eye with aslit beam and calculate the coordinates of the apex of the cornea fromthe difference between the radius of curvature R and the radius ofcurvature R′ of the cornea derived in the tomographic image of theanterior ocular segment.

Further, according to the present invention, there is provided theoptical tomographic imaging apparatus, wherein the detecting device fordetecting a position in a moving part can derive the radius of curvatureR of the cornea by illuminating the center position of the eye withcrossed light of two slit beams and calculate the coordinates of theapex of the cornea from the difference between the radius of curvature Rand the radius of curvature R′ of the cornea derived in the tomographicimage of the anterior ocular segment.

According to the present invention, there can be realized an opticaltomographic imaging apparatus which enables apparatus simplification andcost reduction without reducing accuracy when position information thatthe moving part of an object is moved in an optical axis direction ofmeasuring beam to reduce the deformation of a tomographic image due toposition displacement in the moving part of the object.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining the configuration of an opticaltomographic imaging apparatus measuring system according to a firstembodiment of the present invention.

FIGS. 2A and 2B are diagrams for explaining the configurations of anoptical system for illuminating an anterior ocular segment and anoptical system for observing the anterior ocular segment according tothe first embodiment of the present invention.

FIG. 3 is a diagram illustrating the optical system based on theScheimpflug principle according to the first embodiment of the presentinvention.

FIGS. 4A, 4B, and 4C are diagrams illustrating tomographic images of theanterior ocular segment obtained on an area sensor according to thefirst embodiment of the present invention.

FIGS. 5A, 5B, and 5C are diagrams for explaining the optical system forilluminating an anterior ocular segment according to a second embodimentof the present invention.

FIG. 6 is a diagram illustrating the shape of a cylindrical lensaccording to the second embodiment of the present invention.

FIGS. 7A, 7B, 7C, 7D, 7E and 7F are diagrams for explaining thedifferences between the tomographic images of anterior ocular segmentdue to position displacement between a slit beam and an eye according tothe second embodiment of the present invention.

FIGS. 8A, 8B, 8C, 8D, 8E and 8F are diagrams for explaining thedifferences between the tomographic images of anterior ocular segmentand tomographic images of fundus, due to position displacement betweenilluminating light and an eye and rotation of the eye according to thesecond embodiment of the present invention.

FIGS. 9A, 9B and 9C are diagrams for explaining the optical system forillumination with crossed light according to a third embodiment of thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

First Embodiment

As a first embodiment, a configuration example of an optical tomographicimaging apparatus applied to the present invention will be described.FIG. 1 illustrates a diagram for explaining the configuration of anoptical tomographic imaging apparatus measuring system according to thepresent embodiment. In FIG. 1, the reference numeral 100 denotes aninterferometer unit, and the reference numeral 200 denotes a measuringarm. First, the interferometer unit 100 will be described. The opticaltomographic imaging apparatus of the present embodiment splits lightfrom a light source into measuring beam and reference beam, guides themeasuring beam to an object, guides the reference beam to a referencemirror, and uses return beam of the measuring beam reflected orscattered by the object and the reference beam reflected by thereference mirror to image the tomographic image of the object.Specifically, a low coherence light source emitting a near-infrared rayis used as a light source 101. The light emitted from the light source101 is propagated through an optical fiber 102-1 and is branched intothree optical fibers 102-2, 102-3, and 102-4 by a light branching device103. The optical fiber 102-2 is connected to the measuring arm 200. Theoptical fiber 102-3 is connected to a reference beam arm 110. Thereference beam arm 110 will be described below. The near-infrared rayemitted from the optical fiber 102-3 enters an optical block 112 via acollimate optical system 111, is reflected by a reflection mirror(reference mirror) 113, and reversely follows the optical path so as toenter the optical fiber 102-3. The optical block 112 compensates thedispersion of the optical system of the measuring arm 200. Thereflection mirror 113 is connected to a reflecting position controllingdevice 114. The reflecting position controlling device 114 controls thereflecting position of the reflection mirror 113. As described later indetail, a device 115 for driving the reflecting position controllingdevice drives the reflecting position controlling device to control theoptical path length of the reference beam according to positioninformation detected by a detecting device for detecting a position in amoving part. This can reduce the deformation of the tomographic image ofthe object due to position displacement of the moving part of theobject.

Next, the measuring arm 200 will be described. In the followingdescription of the embodiment, the case that the object is an eye istaken as an example. The light branched by the light branching device103 is emitted via the optical fiber 102-2. The light emitted from afiber end 201 is substantially collimated by an optical system 202. Ascanning device 203 is a galvanomirror which can tilt a mirror surfaceand deflects the incident light. The scanning device 203 is connected toa scanning device controlling circuit 251. In FIG. 1, the scanningdevice 203 is a two-dimensional scanning device having twogalvanomirrors and can perform scanning in a main scanning direction ina sheet surface and a sub-scanning direction in a vertical direction inthe sheet surface. The light scanned by the scanning device 203 passesthrough a beam splitter 208 and forms the conjugate image of the fiberend 201 on an intermediate focusing plane 210 by a focusing lens 209.The light passes through an objective lens 211, an anterior ocularsegment 221 and a pupil 222, and forms an illumination spot on a retina223 of an eye 220 to be inspected. The anterior ocular segment isreferred to as the portion from the cornea to the front surface of acrystalline lens of the eye. Upon in-plane deflection of the scanningdevice 203, the illumination spot is moved on the retina 223. Thereflected light in the illumination spot position reversely follows theoptical path, enters the fiber end 201, and returns to the lightbranching device 103.

Light emitted from a light source 205 for observing an anterior ocularsegment is transmitted through optical systems 206 and 207 forilluminating an anterior ocular segment, is reflected by the beamsplitter 208, and forms an illumination spot on the anterior ocularsegment 221 by the focusing lens 209 and the objective lens 211. Here,the optical system configured in the optical path of the measuring beamand the optical system for illuminating the anterior ocular segment inthe optical tomographic imaging apparatus are partially shared. Thescattered or reflected beams of the illumination spot formed on theanterior ocular segment 221 are focused on an area sensor 232 using anoptical system 231 for observing an anterior ocular segment. Atomographic image of the anterior ocular segment is obtained on the areasensor 232. The obtained image is transmitted to an arithmetic operationprocessing unit 301 via a signal obtaining portion 252. The interferenceof the light returning from the reference beam arm 110 and the measuringarm 200 is detected by a signal detection arm 120.

The signal detection arm 120 emits the light propagated through theoptical fiber 102-4 from the fiber end 121. The light emitted from thefiber end 121 is substantially collimated by an optical system 122. Thesubstantially collimated light enters a diffraction grating 123. Thediffraction grating 123 has a period structure and makes the lightspectral. The spectral light is focused on a line sensor 125 by afocusing lens 124. The line sensor 125 is connected to a detectorcontrolling device 126 and transmits obtained predetermined data to astorage device 302 via the arithmetic operation processing unit 301. Thedata of the storage device 302 is Fourier converted by the arithmeticoperation processing unit 301 to output a tomographic image of thefundus. The tomographic image is outputted on a display device 303. Thepresent embodiment is a low coherent tomography for fundus using theso-called Fourier domain OCT system. In the present embodiment, there isconfigured the detecting device for detecting a position in a movingpart which has the optical system for observing moving part of theobject illuminated by the optical system for illumination with the lightfrom the light source by imaging the same on the area sensor based onthe Scheimpflug principle and detects position information that themoving part of the object is moved in an optical axis direction of themeasuring beam. In other words, the detecting device for detecting aposition in a moving part can detect the positions back and forward themovement of the cornea of the eye in an optical axis direction of themeasuring beam. The position information of the anterior ocular segmentis extracted from the tomographic image of the anterior ocular segment.The position of the reflection mirror 113 of the reference beam arm 110is moved (tracking is performed), whenever necessary, to reduce motionartifacts.

The optical system for illuminating an anterior ocular segment and theoptical system for observing an anterior ocular segment (the opticalsystem for observing moving part) will be described below in moredetail. FIGS. 2A and 2B illustrate diagrams for explaining theconfigurations of the optical system for illuminating an anterior ocularsegment and the optical system for observing an anterior ocular segmentaccording to the first embodiment of the present invention. FIG. 2A is adiagram for explaining the optical system for illuminating an anteriorocular segment. FIG. 2B is a diagram for explaining the optical systemfor observing the anterior ocular segment. Hereinafter, the samereference numerals as those illustrated in FIG. 1 have similar functionsand description thereof will not be repeated. First, the optical systemfor illuminating an anterior ocular segment illustrated in FIG. 2A willbe described. The light source 205 is a light source having a wavelengthof 1300 nm. As the feature of the wavelength range, advantageously, thelight is hard to reach the inside of the eye due to high waterabsorption degree and the anterior ocular segment is easy to beobserved. The illuminating light from the light source 205 istransmitted through the optical systems 206 and 207 for illuminatinganterior ocular segment and is reflected by the beam splitter 208. Thebeam splitter 208 has the property of reflecting light having awavelength of near 1300 nm and of transmitting light having a wavelengthof 800 to 900 nm. The observation of the OCT and the illumination of theanterior ocular segment can be coaxially performed. The property oftransmission and reflection to the wavelengths may be reverse to theabove, in the case, the arrangement of the optical system forilluminating an anterior ocular segment and the optical system forobservation of the OCT would also be reverse. The illuminating lightreflected by the beam splitter 208 forms an illumination spot on theanterior ocular segment 221 by the focusing lens 209 and the objectivelens 211. The depth of focus (DOF) of the illumination spot in avertical direction is desirably more than eye movement of the eye to betracked. The anterior ocular segment 221 is illuminated by the aboveconfiguration of the optical system.

Next, the optical system for observing an anterior ocular segmentillustrated in FIG. 2B will be described. In FIG. 2B, the scattered orreflected beams of the illumination spot formed on the anterior ocularsegment 221 by the configuration illustrated in FIG. 2A are focused onthe area sensor 232 using the optical system 231 for observing ananterior ocular segment. The optical axis of the optical system 231 forobserving an anterior ocular segment is tilted from that of the opticalsystem for illumination. The difference of the position of the eye canbe detected as the difference of the position of the image obtained onthe area sensor 232. This is typically called Scheimpflug principle. Theconfiguration of the optical system of the present embodiment based onthe Scheimpflug principle will be described below.

FIG. 3 illustrates a diagram illustrating the relation of the opticalsystem. The anterior ocular segment is illuminated by the configurationof FIG. 2A. Of the scattered or reflected beams of the illumination spotfrom an anterior ocular segment illumination position I, the beamemitted in the direction of an angle θ is detected. When the principalplane of the optical system is H, the anterior ocular segmentillumination position I and the principal plane H are separated by adistance of s to arrange the optical system. The optical system isarranged in a manner that, when the cross point of the extension planeof the principal plane H and the optical axis of the illuminating lightis C, an angle formed therebetween is a. When the distance between theprincipal plane H and an area sensor surface S at the angle θ is s′ andthe focal length of the optical system is f, s′ is determined by thefollowing equation:

1/s′=1/s+1/f

A magnification β is s′/s. An angle b formed between the extension planeof the area sensor surface S and the principal plane H is determined sothat the extension plane of the area sensor surface S crosses C. Asdescribed above, the above configuration can detect the difference ofthe position of the eye as the difference of the position of the imageobtained on the area sensor.

FIGS. 4A to 4C schematically illustrate the tomographic images of theanterior ocular segment obtained on the area sensor 232. In FIGS. 4A to4C, the positions of the eye are different to one another in an opticalaxis direction (z direction). In a typical optical system in which theoptical axis is coaxial, the difference of the position in the zdirection is detected by the area sensor as a defocused image. On theother hand, in the present invention, the axis of optical system 231 forobserving an anterior ocular segment is shifted. The difference of theposition of the eye is detected as the difference of the position of theimage obtained on the area sensor 232. This enables the positiondetection accuracy in the z direction to be higher than the formermethod.

The optical axis of the optical system 231 for observing an anteriorocular segment may be bent by a prism, not illustrated, to loosen theincident angle of the light upon the area sensor 232. In addition, whenthe optical magnification of the optical system 231 for observing ananterior ocular segment is β, the depth resolution of the OCT is Δ, andthe pitch of the area sensor is p, the following equation is desirablysatisfied:

Δ×β>p

This can perform tracking at the depth resolution Δ of the OCT even inconsideration of the pitch of the sensor. When the eye is moved outsidethe range of the depth resolution Δ of the OCT, the movement is detectedand the position of the reflection mirror 113 is moved, whenevernecessary, to perform tracking.

Here, the depth resolution Δ of the OCT is expressed by the followingequation:

Δ=λ×4×f ² /D ²

where λ is the wavelength of the light source, f is the focal length ofthe eye, and D is the beam diameter incident upon the eye. Thetomographic image of the anterior ocular segment is obtained on the areasensor 232. The obtained image is transmitted to the arithmeticoperation processing unit 301 via the signal obtaining portion 252. Acharacteristic point is extracted from the tomographic image of theanterior ocular segment by the arithmetic operation processing unit 301.For instance, interface information between air and the cornea isextracted by binarization processing and determining the differentialvalue of the image to detect the position of the interface on the areasensor. When the position displacement of the interface from thereference position is δz, the discrepancy information of δz istransmitted to the device 115 for driving the point controlling device.The position of the reflection mirror 113 is then moved by δz by thereflecting position controlling device 114 for performing tracking. Thisis sequentially performed to reduce motion artifacts.

Second Embodiment

In the following, a second embodiment will be described. In the presentembodiment, there will be described an example in which the detectingdevice for detecting a position in a moving part can determine theradius of curvature R of the cornea by illuminating the center positionof the eye with a slit beam and calculate the coordinates of the apex ofthe cornea from the difference between the radius of curvature R and theradius of curvature R′ of the cornea determined in the tomographic imageof the anterior ocular segment. In other words, in contrast to that thespot illuminates the anterior ocular segment in the first embodiment asillustrated in FIGS. 2A and 2B, in the present embodiment, aconfiguration example in which the slit beam illuminates the anteriorocular segment will be described. The layout of the interferometer unit100 is the same as the first embodiment and description thereof will notbe repeated. FIGS. 5A, 5B and 5C illustrate diagrams for explaining theoptical system for illuminating anterior ocular segment according to thesecond embodiment of the present invention. FIGS. 5A, 5B and 5Cillustrate an x cross section, a y cross section, and a z cross section,respectively. The illuminating light from the light source 205 istransmitted through the optical system 206 for illuminating anteriorocular segment and enters a cylindrical lens 207′. FIG. 6 illustratesthe shape of the cylindrical lens 207′. The cylindrical lens has theproperty of refracting the light on the x cross section and of notrefracting the light on the z cross section. The illuminating lightdifferently refracted based on each cross section is reflected by thebeam splitter 208. The illuminating light reflected by the beam splitter208 illuminates the anterior ocular segment 221 in slit shape by thefocusing lens 209 and the objective lens 211. The illuminating light isspot-like seen on the x cross section. The illuminating light is notfocused seen on the y cross section and illuminates the wide area. Thescattered or reflected beams of the slit illuminating light formed onthe anterior ocular segment 221 by the configurations of FIGS. 5A, 5Band 5C are focused on the area sensor 232 using the optical system 231for observing anterior ocular segment. An optical axis of the opticalsystem 231 for observing an anterior ocular segment is tilted from thatof the optical system for illumination. The difference of the positionof the eye can be detected as the difference of the position of theimage obtained on the area sensor 232 based on the Scheimpflugprinciple.

The diagrams for explaining the differences between the tomographicimages of the anterior ocular segment due to position displacementbetween the eye and the slit beam according to the present embodimentare illustrated. FIGS. 7A and 7B schematically illustrate thetomographic image of the anterior ocular segment obtained on the areasensor 232. FIG. 7A illustrates the illuminating state of the slit beamon the anterior ocular segment. FIG. 7B illustrates the tomographicimage of the anterior ocular segment obtained on the area sensor 232.Hereinafter, the position of FIGS. 7A and 7B will be called a normalposition. The difference of the position of the eye is detected as thedifference of the position of the image obtained on the area sensor 232,which is the same as the first embodiment. Like FIGS. 7C and 7D andFIGS. 7E and 7F, when the eye is moved in a horizontal direction (the xor y direction) to the optical system, tracking in the z direction canbe performed by taking the shift amount into consideration.

FIGS. 7C and 7D illustrate the case that the eye is moved in the ydirection to the optical system. When the eye is moved in the ydirection to the slit beam, the plane sectioning the anterior ocularsegment is shifted. Therefore, when the anterior ocular segment image ofFIGS. 7C and 7D is compared with the anterior ocular segment imagedetected in the normal position of FIGS. 7A and 7B, the curvatures ofthe cornea are different. Then, the difference of the curvature of thecornea can be used to calculate the amount of movement in the ydirection so that the top position can be determined. The procedure willbe described below. The radius of curvature of the cornea herein is theapproximate curvature of the cornea determined by the tomographic imageof the anterior ocular segment. The radius of curvature of the corneacan be determined by determining the position coordinates of the corneaon at least three points and by fitting using the least squares method.

First, the radius of curvature of the cornea is measured. The anteriorocular segment image is imaged on the area sensor 232 using the opticalsystem 231 for observing an anterior ocular segment in the positionwhere the slit beam illuminates the center position of the eye, and theradius of curvature is determined from the image. Alternately, if thecenter position is ambiguous, the light source 205 is shifted in avertical direction to the optical axis and the above operation ofdetermining the radius of curvature is performed for each shift, and thelight source 205 is set in the position where the radius of curvature ismaximum, so that the radius of curvature of the cornea can also bemeasured. When the cornea is assumed to be spherical and the coordinatesof the apex of the cornea in the normal position are (x, y, z)=(0, 0,0), the shape of the cornea can be expressed by the following equation:

x ² +y ²+(z−R−z1)² =R ²   Equation (1)

where R is the radius of curvature of the cornea, and z1 is the amountof movement of the eye in the z direction. In the normal position ofFIGS. 7A and 7B, the anterior ocular segment is sectioned at y=0. Then,Equation (1) becomes

x ²+(z−R−z1)² =R ²   Equation (2)

The relation is imaged on the area sensor 232. Therefore, tracking inthe z direction can be performed by reading z1. On the other hand, whenthe optical axis is shifted by y1 in the y direction like FIGS. 7C and7D, the anterior ocular segment is sectioned at y=y1. Then, Equation (1)becomes

x ²+(z−R−z1)² =R ² −y1² =R′ ²   Equation (3)

The radius of curvature of the image on the area sensor 232 becomesR′=√(R²−y1 ²).

the amount of movement y1 ² in the y direction from the normal positioncan be determined by using the radius of curvature calculated from theimage on the area sensor 232 and the first measured radius of curvatureR of the cornea. However, as understood from y1 ² in the equation, here,it cannot be found whether y1 is a positive value or a negative value.However, a plurality of slit beams can be incident upon the eye so thatthe absolute value of y1 is found. From the above description, inconsideration of the amount of movement y1 in the y direction, z1 iscalculated so that tracking in the z direction can be performed.

FIGS. 7E and 7F illustrate the case that the eye is moved in the xdirection to the optical system. When the eye is moved in the xdirection to the slit beam, the image is shifted in the x direction inthe area sensor 232 so that the amount of movement in the x directioncan be calculated from the image position. In addition, the amounts ofmovement in the x direction and the y direction can be independentlydetermined. Therefore, even when the eye is moved in both thedirections, the coordinates of the apex of the cornea are calculated sothat tracking in the z direction can be performed. Further, even whenthe eye is rotated relative to the optical system, tracking in the zdirection can be performed by taking the shift amount intoconsideration.

FIGS. 8A, 8B, 8C, 8D, 8E and 8F illustrate diagrams for explaining thedifferences between the tomographic images of the anterior ocularsegment and the tomographic images of the fundus due to positiondisplacement between the illuminating light and the eye and rotation ofthe eye according to the present embodiment. FIGS. 8A, 8B and 8Cillustrate that the eye is shifted in the y direction to the opticalsystem and FIGS. 8D, 8E and 8F illustrate that the eye is rotated in they direction. FIGS. 8A and 8D illustrate the optical paths of theilluminating light on the fundus. FIGS. 8B and 8E illustrate thetomographic images of the anterior ocular segment. FIGS. 8C and 8Fillustrate the tomographic images of the fundus. Comparing the imagewhen the eye is shifted with the image when the eye is rotated, theilluminated area of the anterior ocular segment, not illustrated, ishardly changed in the tomographic image of the anterior ocular segment,but the illuminated area of the fundus is largely changed in thetomographic image of the fundus. This is because, whereas in shifting ofthe eye, almost the same position of the illuminated area of the fundusis illuminated, by contrast in the rotation of the eye, the beam withthe angle of view enters the eye to shift the illuminated area of thefundus. Then, tracking in the z direction can be performed by detectingthe shift of the illuminated area of the fundus, and calculate arotation angle which is converted to an optical path length. Thecorrespondence of the conversion of the rotation angle to the opticalpath length need be performed in advance.

Third Embodiment

A third embodiment will be described. In the present embodiment, therewill be described an example in which the detecting device for detectinga position in a moving part can determine the radius of curvature R ofthe cornea by illuminating the center position of the eye with crossedlight of two slit beams and calculate the coordinates of the apex of thecornea from the difference between the radius of curvature R and theradius of curvature R′ of the cornea determined in the tomographic imageof the anterior ocular segment. In other words, in the presentembodiment, there will be described a configuration example in which theslit beams cross each other (hereinafter, called crossed light) forillumination will be described. The layout of the interferometer unit100 is the same as the first embodiment and description thereof will notbe repeated. FIGS. 9A, 9B and 9C illustrate diagrams for explaining theoptical system for illumination with the crossed light according to thepresent embodiment. FIG. 9A illustrates the x cross section, FIG. 9Billustrates the y cross section, FIG. 9C illustrates the z crosssection. The light source 205-a is used for the slit beam extended inthe x-axis direction. The light source 205-b is used for the slit beamextended in the y-axis direction. The optical systems 206-a, 207′-a,208-a, 209, and 211 for the slit beam extended in the x-axis directionare the same as FIGS. 5A, 5B and 5C and description thereof will not berepeated.

The optical system for the slit beam extended in the y-axis directionwill be described. The optical paths indicated by the dashed lines ofFIGS. 9A to 9C are optical paths of the slit beam extended in the y-axisdirection. The illuminating light from the light source 205-b istransmitted through the optical system 206-b for illuminating ananterior ocular segment and enters the cylindrical lens 207′-b. Thecylindrical lens 207′-a has the property of not refracting the light onthe x cross section and of refracting the light on the z cross section.The illuminating light differently refracted based on each cross sectionis reflected by the beam splitter 208-b. The illuminating lightreflected by the beam splitter 208 illuminates the anterior ocularsegment 221 in slit shape by the focusing lens 209 and the objectivelens 211. The illuminating light is not focused seen on the x crosssection and illuminates the wide area. On the other hands, theilluminating light is illuminating spot-likely seen on the y crosssection. In other words, the illuminating light is the slit beamextended in the y-axis direction.

The scattered or reflected beams of the cross illuminating light formedon the anterior ocular segment 221 by the configurations of FIGS. 9A, 9Band 9C are focused on the area sensor using the optical system forobserving an anterior ocular segment. At least two area sensors arenecessary using the optical system for observing an anterior ocularsegment. The directions to which the optical axes are tilted need bevertical to each other. The area sensor is also provided in a verticaldirection in the sheet surface using the optical system for observing ananterior ocular segment illustrated in FIG. 2B. The optical system forobserving an anterior ocular segment shifted in the y direction and thearea sensor focus the scattered or reflected beams of the illuminatinglight extended in the x direction, of the crossed light illustrated inFIG. 9. The optical system for observing an anterior ocular segmentshifted in the x direction and the area sensor focus the scattered orreflected beams of the illuminating light extended in the y direction,of the crossed light illustrated in FIG. 9.

The method of extracting the coordinates of the apex of the cornea isthe same as the second embodiment and description thereof will not berepeated. By the above configuration, the tomographic image of theanterior ocular segment can be obtained on each of the x cross sectionand the y cross section. The accuracy is increased because the amountsof movement in the x direction and the y direction are determined bothfrom two directions. In addition, the coordinates of the apex of thecornea can be extracted even if the shape of the cornea is asymmetrical.The crossed light having different wavelengths may be emitted. The lightsources 205-a and 205-b may be light sources having differentwavelengths to obtain the tomographic image of the anterior ocularsegment.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-011615, filed Jan. 22, 2009, which is hereby incorporated byreference herein in its entirety.

1. An optical tomographic imaging apparatus which splits light from alight source into measuring beam and reference beam, guides themeasuring beam to an object, guides the reference beam to a referencemirror, and uses return beam of the measuring beam reflected orscattered by the object and the reference beam reflected by thereference mirror to image the tomographic image of the object,comprising: a reflecting position controlling device for controlling thereflecting position of the reference mirror; a detecting device fordetecting a position in a moving part which has an optical system forobserving moving part of the object illuminated by an optical system forillumination with the light from the light source by imaging the same onan area sensor based on the Scheimpflug principle and detects positioninformation that the moving part of the object is moving in an opticalaxis direction of the measuring beam; and a device for driving thereflecting position controlling device to control the optical pathlength of the reference beam according to the position informationdetected by the detecting device for detecting a position in a movingpart and reducing the deformation of the tomographic image of the objectdue to position displacement of the moving part of the object.
 2. Theoptical tomographic imaging apparatus according to claim 1, wherein theobject is an eye, the moving part of the object is an anterior ocularsegment, and the detecting device for detecting a position in a movingpart can detect the positions back and forward the movement of thecornea of the eye in an optical axis direction of the measuring beam. 3.The optical tomographic imaging apparatus according to claim 1, whereinwhen the depth resolution of the optical tomographic imaging apparatusis Δ, the optical magnification of the optical system for observingmoving part is β, and the pitch of the area sensor is p, the followingequation is satisfied:Δ×≈>p
 4. The optical tomographic imaging apparatus according to claim 1,wherein the optical system configured in the optical path of themeasuring beam and the optical system for illuminating the moving partof the object in the optical tomographic imaging apparatus are partiallyshared.
 5. The optical tomographic imaging apparatus according to claim2, wherein the detecting device for detecting a position in a movingpart can derive the radius of curvature R of the cornea by illuminatingthe center position of the eye with a slit beam, and can calculate thecoordinates of the apex of the cornea from the difference between theradius of curvature R and the radius of curvature R′ of the corneaderived in the tomographic image of the anterior ocular segment.
 6. Theoptical tomographic imaging apparatus according to claim 2, wherein thedetecting device for detecting a position in a moving part can derivethe radius of curvature R of the cornea by illuminating the centerposition of the eye with crossed light of two slit beams and calculatethe coordinates of the apex of the cornea from the difference betweenthe radius of curvature R and the radius of curvature R′ of the corneaderived in the tomographic image of the anterior ocular segment.